Why Are High Elevations in the Northeast United States Warming Slower than Surrounding Low Elevations? First Steps toward Testing the Vertical Airmass Hypothesis

While low elevations are always in the boundary layer, higher elevations can experience considerable time in other air masses besides the boundary layer, including the entrainment zone, free troposphere, and localized surface boundary layers. Since each of these air masses can have distinct thermal, moisture, chemical, and cloud properties, it follows that the type of air mass exposure could be a significant driver of high elevation climate, air quality, and elevation dependent warming. Furthermore, it could help explain why a minority of mountain ranges (e.g., northern Appalachians, Niwot Ridge) are seeing slower warming than surrounding lower elevations, contrary to global circulation model projections. Discrimination of air mass exposure type in montane environments can be a challenge and is not commonly performed, but is necessary to determine its importance on high elevation climate and air quality.

On 19 August 2016, an intensive observation period (IOP) was performed to capture a detailed evolution of the vertical structure of the lower troposphere on and around Mount Washington (1917 m asl). In addition to the regular meteorological observations that are taken at the summit (temperature, relative humidity, wind, pressure, sky coverage, cloud type, etc.) and several mesonet sites on the east and west slopes (temperature, relative humidity, and wind), radiosondes were launched from the base of the Auto Road from pre-sunrise to post-sunset. Concurrently, water vapor stable isotopes were measured by a water vapor isotope analyzer that was driven in a pick-up truck up and down the Auto Road.

During the early morning of the IOP, a 15-20 m s^-1wind forced residual layer air up the slopes to the summit from an altitude as low as 1400 m asl. Meanwhile, radiosonde temperature and dewpoint profiles indicate the boundary layer top was below the summit elevation in the free atmosphere. Winds diminished by 1000 Eastern Standard Time (EST), and highly variable summit water vapor isotope ratios and dewpoint values indicate the entrainment zone descended upon the summit. Around 1200 EST, the convective boundary layer grew through the summit elevation as suggested by a rapid increase in dewpoint, base-to-summit lapse rates at or exceeding dry adiabatic, and homogenizing of wind direction at sites above treeline. This IOP reveals important thermal and air mass height differences between the free atmosphere and along mountain slopes and provides guidance on methods to observe these differences. This research suggests that most mountain ranges around the world have an elevational range that experiences regular variability in exposure to the boundary layer, entrainment zone and free troposphere that help to characterize local temporal changes in climate and air quality.